Conventional hollow injection-molding methods of forming a hollow portion in molten resin in a mold cavity by introducing a pressurized fluid, wherein the pressurized fluid is directly introduced into molten resin in a mold cavity and the pressurized fluid in the hollow portion is discharged through the introducing passage for the pressurized fluid, and a pressurized fluid introducing apparatus therefor are disclosed in Examined Japanese PatentPublications Nos. SHO. 48-41264, and SHO. 59-19017, and U.S. Pat. No. 4,740,150.
Examined Japanese PatentPublication No. SHO. 48-41264 discloses a hollow injection-molding method and a pressurized fluid introducing and discharging apparatus therefor wherein, as shown in FIG. 11, a thin cylindrical projection 102 having a pressurized fluid passage 101 therein is disposed so that it can be projected into and retracted from a mold 103. While the projection 102 is projected into the mold 103, molten resin is injected and a pressurized fluid is introduced from the pressurized fluid passage 101, thereby forming a hollow portion. After cooling and solidification, the pressurized fluid is discharged from the hollow portion through the pressurized fluid passage 101. The projection 102 is retracted from the mold 103, and then the mold article is removed.
Examined Japanese PatentPublication No. SHO. 59-19017 discloses a hollow injection-molding method and a pressurized fluid introducing and discharging apparatus therefor wherein, as shown in FIG. 12, a valve 107 operated by a cylinder 106 to open and close the pressurized fluid passage 101 is disposed in the intermediate portion of a molten-resin passage 105 through which injected molten resin is guided to a mold cavity 104. A pressurized fluid is introduced and discharged through the valve 107 and the pressurized fluid passage 101.
U.S. Pat. No. 4,740,150 discloses an injection-molding method and a pressurized fluid introducing and discharging apparatus therefor wherein a pressurized fluid is introduced into molten resin injected into a mold cavity, through a non-return valve and a nozzle having a pressurized fluid introducing passage which can be communicated with the mold cavity. After the resin solidifies, the nozzle is retracted to open the valve port so that the pressurized fluid in the resin is discharged to the atmosphere.
However, these hollow injection-molding methods and pressurized fluid introducing and discharging apparatuses therefor have following problems.
The hollow injection-molding method and the pressurized fluid introducing and discharging apparatus therefor which are disclosed in Examined Japanese PatentPublication No. SHO. 48-41264 have problems as follows: When molten resin is injected, the molten resin flows back into the pressurized fluid passage 101 in the projection 102, thereby causing the pressurized fluid passage 101 to be clogged. In order to prevent the backflow of molten resin, it may be considered that the bore diameter of the pressurized fluid passage is reduced. However, the time to discharge the pressurized fluid from the hollow portion then increases, thus increasing mold cycle time and reducing mold efficiency.
The hollow injection-molding method and the pressurized fluid introducing and discharging apparatus therefor which are disclosed in Examined Japanese Patent Publication No. SHO. 59-19017 have problems as follows: When the valve 107 is opened during the introduction of the pressurized fluid, the molten resin flows back into the pressurized fluid passage 101 through the opened valve 107, thereby causing the pressurized fluid passage 101 to be clogged. Furthermore, the pressurized fluid in the introduction process may fail to be introduced into the molten resin and leak out through the molten resin and the inner surface of the mold 103 to the atmosphere. As a result, a hollow portion can not be formed or the pressure of the pressurized fluid in a hollow portion is not maintained at a required level, thereby causing a poor moldability.
The hollow injection-molding method and a pressurized fluid introducing and discharging apparatus therefor which are disclosed in U.S. Pat. No. 4,740,150 have problems as follows: The apparatus has the nozzle functioning as a valve member and the valve seat, and in which the pressurized fluid introducing passage is formed in the nozzle and a pressurized fluid discharging passage is formed outside the nozzle. Such a construction causes the valve member and valve seat to have a complex structure, and since the pressurized fluid introducing passage is separated from the discharging passage, the structure of the pressurized fluid introducing and discharging apparatus is complicated. Furthermore, since the introducing passage is closed by the non-return valve, except when the pressurized fluid is introduced, the mechanism of operating the non-return valve is further complicated.
The invention was made in view of the above-mentioned problems, and has an object of preventing the backflow of molten resin into a pressurized fluid passage and performing sure introduction and rapid discharging of the pressurized fluid.